Refractive Indices of Silsesquioxanes with Various Structures

In order to provide reliable encapsulants for a high-performance LED, silsesquioxane is one of the most promising materials. However, the refractive indices of conventional silicone materials are relatively low and improvement is necessary. For the aim of developing silicone materials with high refractive index, we investigate the relationship of structures and refractive indices. In this paper, a series of alkyl-substituted silsesquioxanes with various structures (linear, cyclic, ladder, and cage) were newly synthesized and spectral and optical properties were investigated. Thus, linear and cyclic siloxanes were prepared from respective silanols. Two novel tricyclic laddersiloxanes (ladder-type silsesquioxanes with defined structure) were synthesized from cyclic silanols. Cage octasilsesquioxanes with same substituents were also prepared by applying the new method. The structures of the products were identified and their refractive indices were measured. As a result, cyclic siloxanes showed slightly higher values than linear or monomeric siloxanes. By adding more rings, refractive index values increased. Among the compounds we investigated, a tricyclic laddersiloxane comprising 6,8,6-membered rings showed the highest refractive index and Abbe number.